This invention relates generally to X-ray apparatus and is concerned more particularly with an X-ray film cassette holder having means for centering a film cassette in the holder.
In some X-ray diagnostic procedures, such as examinations of the chest, for example, a selected portion of a patient's body is positioned adjacent an aligned film in a vertically supported cassette. Generally, the cassette is slidably inserted into one side of an upright holder, such as a wall holder, for example, which has means for positioning the cassette longitudinally as desired. An X-ray source is operatively disposed for directing an X-ray beam through the selected portion of the patient and onto the aligned film. As a result, the beam is modified by the internal structure of the selected portion and produces a shadow image thereof on the film.
In order to minimize exposure of the patient to unnecessary X-radiation, it is required that the irradiated portion of the patient correspond substantially to the area of the aligned film. Since films of various sizes may be installed in the described holder, it is desirable that adjustable means be provided for controlling the cross-sectional size of the X-ray beam. Consequently, there may be disposed between the X-ray source and the patient a beam collimator device having adjustable shutters made of X-ray absorbent material for defining an aperture through which the beam passes. Thus, by adjusting the shutter aperture the diverging X-ray beam is provided with a cross-sectional size which conforms substantially to the area of the film, at the plane of the film.
However, even when the incident cross-sectional size of the X-ray beam is adjusted as described, the patient still may be irradiated unnecessarily due to the X-ray film cassette not being centered transversely in the holder. Thus, some of the X-rays passing through the selected portion of the patient may not impinge on the off-center film and, consequently, will serve no useful purpose. Generally, it is left to the operator or X-ray technician to center the film cassette in the holder visually, as by aligning a scribe mark on the cassette with a notch in the holder, for example. Also, there may be occasions when it is preferable that the patient not be required to move while taking a plurality of exposures.
Therefore, it would be advantageous to provide an X-ray cassette holder with automatic means for transversely centering a film cassette installed in the holder.